Questions on planning a dive/surface interval

[Sweet Red]

"He was therefore, under PADI Rules, only qualified to dive to 18 metres."

In Kauai last week, on my third dive since certification, one DM took led our group of six. I was loosley assigned with a buddy, but we all stayed with the DM. The other five were very experienced divers and I made it clear that I had only previously dove three days before in the Big Island. On the first dive we went 85 feet (about 25m) for 34 minutes (with 50 minutes SI) and on the second dive went 60 feet for 38 minutes. I found this out after we got back to the shop where a print out was provided.

I got back to my hotel that afternoon to complete my dive book and found that the dives were off of my dive table. Consequently, I never completed the log. (I my naivate' I just assumed I was tired and reading the book wrong - even though I aced the tests in this area.)

It appears that based upon the above quoted statement that I should not have gone so deep. I will be blunt and confess that I was not watching my depth gauge closely since I was following the DM, but even if I had, would not have known what to do about it - assuming I knew that I had to stay above 18m in the first place.

Also, I was the third to ascend, with pre-dive instructions for a safety stop at 20 feet. The DM pointed me in the direction of the line, clearly visible, made certain I began my ascent, and the led the other three back out. I think - think - he stayed relatively close by and within range of my visibility, but the current was strong and I hung on tightly to the safety line while asscending.

Is the depth of an additional 6 meters really a big deal, assuming SI is adequate? The again, how do you know the appropriate SI if it's off the charts??? Could you please share your thoughts? Thank you.

P.S. I spent hours researching reputable dive shops before the trip, which I confirmed with others once I got to Poipu.

 

[kidspot]

Most likely the reason the dive didn't fit your tables was because he was doing a multi-level dive using a computer... as to the depth, normally before a dive they will give you the max depth of the anticipated dive and it would be up to you to determine if it is within your abilities... The 18m depth limit would probably be more accurately called a recommended depth limit... but you are the one responsible for your own safety as a diver...

As to the ascent, while it is quite common to leave a diver at the upline, it is debatable if this is a sound practice or not, especially if you were not with a buddy at the time...If with a buddy, then it's not really a problem as you and your buddy, as open water divers, are suppossed to be qualified to dive on your own...

Much of this you will have to determine your comfort with and tolerance of in the future, only understand that it is common in places like Hawaii...

Aloha, Tim

 

[abysmaldiver]

Well I think you just learned what a "trust me dive" is. My first recommendation would be to get a dive computer and use it through out your dive. You asked if the addtional 6 meters is a big deal, yes and no. You did pass your recommended depth limit for your training but if you were comfortable it is not the end of the world. I would like to see your profile for that dive because if you spent the entire dive at 85' you were probably in deco.

 

[H2Andy]

On the first dive we went 85 feet (about 25m) for 34 minutes (with 50 minutes SI) and on the second dive went 60 feet for 38 minutes. I found this out after we got back to the shop where a print out was provided.

like kidspot said, your DM was calculating your dive using his computer.

(what's wrong with this statement? more on this from others, i am sure)

were you using air or another mix? let's assume air for now. per my PADI tables, the max at 90 feet on air is 25 minutes, counted from entrance to the water to
commencing the ascent from the maximum depth (not total in-water time).

if you left 85 feet after 25 minutes, and spent 13 minutes at shallower depths, you
were ok (total dive time 34 minutes). hopefully, you spent most of that time above
33 feet for maximum offgassing.

after that dive, you are in pressure group Q. after 55 minutes, you are in pressure
group G. Your actual bottom time is 34 minutes. Again, assuming you left
60 feet after 34 minutes in the water and spent the other four minutes at shallower
depths, you would be ok.

what you didn't take into account was that you don't use your total dive time
but only your bottom time (from surface to deepest depth) to calculate your dive
profile.

[note: the definition of bottom time i am using is "time from begining of
descent to beginning of ascent". any time you spend shallower than your
deepest depth, you will be off-gasing for that max depth so long as you
continue a slow and gradual ascent. i use a three-minute stop at
1/2 max depth, then a slow ascent to 30 feet with 1 minute stop, slow
ascent to 20 feet with 1 minute stop, 3 minute stop at 15 feet, 1 minute
stop at 10 feet, and slow ascent]

 

[kidspot]

... your DM was calculating your dive using his computer...

there's the problem ... every diver needs their own computer if diving by one ... and *he* was planning *your* dive ... each diver should know the plan ahead of time, and agree with/follow it if diving this way... each diver always retains responsibility for their own safety and dive plan... Even on a "trust me" dive you stillneed to be responsible for your own safety.

Aloha, Tim

 

[Charlie99]

A problem common with multilevel dives (which I assume you were doing) is that most divers without computers don't know how to calculate them, and therefore end up doing a "trust me" dive in that they don't really know their decompression status.

H2Andy's comments above about bottom time ending when you start to ascend from the maximum depth apply only when starting a DIRECT ascent to the safety stop or surface (I don't have PADI OW book handy, so the exact words might be a bit different). If you spent a bunch of time at 85', then meandered along at 60', then headed on up to 40', then the PADI RDP, which is designed for square profile dives isn't going to give you a decent answer. You are stuck trusting the DM's computer. Not a good idea.

A better dive op would have at least mentioned the multilevel plan, such as "20 minutes at 85', then 10 minutes at 60', then the remainder of the dive above 40' " Unless you have a PADI wheel with you, or have some other method of planning a multilevel dive, you are still blindly following the DM, but at least you know what the plan is.

 

[duckster3d]

Sweet Red, First let me start by saying take this as a learning experience.

You are a certified diver therefore responsible for your own dive plan, dive depth, and dive time. You should plan your dive before you do the dive and not find out after you get back to your hotel room that you were off your tables. I have been teaching diving since 86, and when i started teaching we certified divers to go to 130 right out of class. The new standards now recommend 60' and take an advance course before going deeper. IE: put another dollar in. anyways you should have listened to the briefing and decided if you could make the dive, tracked your own depth and time, came up within the no deco limits, and done a safety stop, all things you should have learned from class. This sport is one that can kill you. DO NOT TAKE IT LIGHTLY!!!

you and only you are responsible for your safety.

ssi aowi 3165

 

[TSandM]

Sweet Red -- welcome to the "real world" of diving.

I empathize with your question. Last summer, after I learned to dive, I posted similar questions, because it was driving me crazy that I couldn't use the tables to sort out the dives I was doing. If you go down to 130 feet for five minutes to look at garden eels, then come up to 60 feet for a while, then up to 45 for a few minutes, and then ascend to the boat . . . what pressure group are you? Using the max depth and bottom time rule, you just spent an hour at 130 feet, which puts you so far into deco you'll never come out Of course, that's not what you did, and a computer knows it, but the RDP tables don't.

You can learn to use the wheel, which is a multi-level planning tool, but even that won't help you very much if you are doing terrain-based diving, where you go up and down some and gradually ascend.

You are being diligent, and that's good, but you honestly haven't been given the tools to analyze the actual dives you are doing. And although dive briefings will usually tell you the max depth you will reach, they are (in my experience) rarely detailed enough to allow you to sit down with a wheel and try to plan out how long you can stay at each depth, or even take the dive you already DID and try to figure out how long an SI you need to be safe for the next dive as described, if it's even been described.

You can go quite crazy trying to deal with this. Or you can start studying decompression theory and start putting together a rational construct in your mind for how this all works . . . how do you load nitrogen, how do you offgas, what do safe decompression curves look like, and how can you relate that to the tables, to what your computer tells you, and what you have been taught about how to handle ascents and stops.

You CAN dive the tables, but your dives will be so much shorter than anybody else's that it will be very unsatisfying to all concerned. You can buy a computer, which will allow you to do the more complicated profiles, but it's an awfully good idea to learn how the computer is doing what it is doing, so that you have some idea of what to do if it goes haywire or if the information it's giving you makes no sense.

Anyway, just wanted to tell you that you aren't the only person who has gotten frustrated and confused trying to deal with the cognitive dissonance of what you were taught and what everybody seems to do

BTW, if the DM knew you were on your third post-certification dive, and left you on the upline to ascend by yourself, I don't think much of him.

 

[alcina]

And although dive briefings will usually tell you the max depth you will reach, they are (in my experience) rarely detailed enough to allow you to sit down with a wheel and try to plan out how long you can stay at each depth, or even take the dive you already DID and try to figure out how long an SI you need to be safe for the next dive as described, if it's even been described.

In this case, the certified diver needs to slow down and ASK more questions. It's not OK to just jump in. The certified diver is the ONLY one responsible for the dive. If information is missing from the brief (and likely it will be - after all, this is a "brief"), the certified diver needs to ask for whatever is needed to be able to plan a dive within his/her limits, training, experience, comfort level etc.

If one does not know, or is not comfortable in, how to plan a multilevel dive some additional training/study etc is the way to go...before the dive

 

[Doc Intrepid]

"Is the depth of an additional 6 meters really a big deal, assuming SI is adequate"?

Yes. Gas loading varies with respect to depth. Moreover, numerous things can happen that may task-load new divers to the point that they panic. If these things occur at significant depths, the time you have to respond appropriately is decreased. This is why there are initial limits placed on how deep brand new divers ought to go. But it is your responsibility, not the divemasters, to ensure that the limits are adhered to.

"Then again, how do you know the appropriate SI if it's off the charts???"

You don't. Your best bet - assuming you've exceeded your tables on Dive A, but you don't know by how much, nor what surface interval is adequate - is to thumb Dive B. There will always be another dive, another day.

"Could you please share your thoughts?"

As the others have noted, you conducted a "trust me" dive. You essentially put your own safety and well-being into the hands of another diver, whom you 'assumed' would have your best interest consistently in mind. This is really unwise, for a large number of reasons. It would be a good thing if you learned from this and made it a point not to do any more trust me dives.

You survived the experience, you likely enjoyed the dives, it was overall a fun trip, and now you're learning valuable lessons. If you're going to dive tables, don't dive with computer divers - their profiles are likely to consistently be different from yours. If on the other hand you want to dive with computer divers, get a computer. You can still plan dives with a computer. Don't let the computer dictate the dive. But your profiles are likely to be far more compatible with the rest of the group.

Welcome to ScubaBoard!

Doc